1. Field of the Invention
This invention relates to high temperature multicell electrochemical storage batteries.
A high temperature battery operates at a temperature above ambient. A typical example of such a battery is one using a molten alkali electrode where in order to operate, the temperature of each cell must be above the melting point of the alkali metal. The invention has particular application to batteries, such as sodium sulphur batteries which are capable of high rates of charge and discharge.
A high temperature storage battery has to be provided with heating means so that it can initially be raised to the required temperature. However, when a battery is charging or discharging, due to the internal resistance, heat is developed inside each cell and hence, during normal operation the temperature of the battery would rise unless heat is removed. The normal practice is to allow the excess heat to be dissipated through the cell wall and, in the case of a multicell battery, through the battery housing.
Since batteries may be used intermittently they must not be cooled too quickly as they might fall below the minimum temperature for operation. It has therefore been proposed to provide insulation around individual cells or around a battery of cells to prevent or at least delay cooling of the cells. The provision of such thermal insulation however inhibits effective cooling if the cell should overheat for any reason.
2. Prior Art
These considerations lead to designing the housing wall as a compromise between the requirements for heat transmission and thermal insulation. In U.K. Specification No. 2020087, which is an example of such a compromise, insulation for a multicell storage battery is provided by a vacuum compartment within the housing wall and, to deal with the problem of possible overheating, hydrogen is admitted into the vacuum compartment to give thermal conduction. The hydrogen is normally absorbed on a material such as palladium. To release the hydrogen, the palladium has to be heated to a temperature above the operating temperature of the cell. To render the insulation system re-effective, the hydrogen storage system must be cooled down so that the gas is re-absorbed. Such an arrangement however still requires that heat must be transferred through the battery housing. This limits the number of cells which can be provided within the housing since it is undesirable to have any cell remote from the heat transfer means. Furthermore if a large battery is to be formed of a plurality of multicell modules with heat transfer through the housing for each module, then provision has to be made for heat transfer from the outer surfaces of the various individual modules, preventing close stacking of the modules.